In many applications there is a need to focus or concentrate all of a particle beam's energy on target volumes. In other cases only a portion of the total beam energy is useful for effecting the change desired and the remainder is waste. The waste is heat, which can be difficult and expensive to deal with. Disposing of the waste heat can be so difficult or expensive that a particular application may be impractical or impossible.
For example, 99Mo, which is the parent of 99mTc, an isotope widely used for medical diagnostic purposes, can be produced by the photonuclear transmutation of 100Mo. The process requires bremsstrahlung to interact with 100Mo. “bremsstrahlung” (meaning braking radiation) is the radiation which is emitted when electrons are decelerated or braked when they are fired at a target. Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high enough, that radiation is in the x-ray region of the electromagnetic spectrum. Bremsstrahlung is characterized by a continuous distribution of radiation which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons is increased. The more intense the bremsstrahlung, the higher the specific activity of the 99Mo (in Curies/gram). To produce bremsstrahlung of sufficient intensity to create photonuclear transmutation of 100Mo requires very high electron beam intensity at very high kinetic energy. Providing such a high electron beam intensity at high kinetic energy is readily achievable.
However, while producing a beam of sufficient intensity and energy is readily achievable, the means to deliver the necessary intensity of bremsstrahlung to a material intended for photonuclear transmutation has not heretofore been practicable. To extract a high energy, high power, and high areal power density electron beam from its acceleration environment (which is high vacuum), through a vacuum barrier, and through atmosphere to a bremsstrahlung converter suffers several impediments. First, in high power operation, only about half the beam power is converted to useable bremsstrahlung; the remainder is waste heat. Due to the rate of power absorption in the vacuum barrier and the converter, this waste heat will destroy most practical materials of which the vacuum barrier and the converter can be made.
It is, therefore, desirable to provide an improved means to extract a high power density particle beam from a particle accelerator for application to a material.